Why do trees change colour?

Trees are the stars of the show in autumn, with glorious colours and shedding leaves. But why does this happen?

Changing colours

The colours you see in the leaves are all down to three pigments: chlorophylls, carotenoids and anthocyanins.

Chlorophyll is responsible for the green colour. Chlorophyll is mostly present during summer months, except in evergreen plants, and are important in the process that plants use to make food from sunlight (photosynthesis).

Carotenoids make yellows, oranges and browns. They are in the leaves all the time, but we only see them in the autumn.

Anthocyanins make leaves red and in most trees are only made in the autumn.

In the autumn, as the temperature drops and the amount of sunlight decreases, a tree makes less chlorophylls, and the ones in the leaves begin to break down. As this happens, we can see the carotenoids which have been hiding for months. Some leaves also make anthocyanins and so red begins to appear as well.

Crunchy carpets

Of course the colour of leaves is not the only great thing about autumn – crunchy leaves underfoot are also a key part of the changing season. But why do leaves fall off deciduous trees?

It’s basically all about protection – the leaves won’t survive the freezing winter, so the tree needs to get rid of them to protect itself and to save water and energy.

It makes a special layer of cells at the end of the leaf’s stem to disconnect it. When this happens, the leaves are only loosely connected to the tree, so they fall off or are blown off in the wind, and end up as a crunchy carpet. This process is known as abscission.

Evergreens stay green all year photosynthesising. They produce special anti-freeze properties which help to prevent leaves getting damaged by low temperatures.